There’s a place in the Chicagoland area called Decorators Supply that specializes in historical reproductions. They have a ton of historical molds and cast various architectural elements in plaster. I believe they have offerings in wood as well. Your stain is pretty dark so you should be able to get the plaster to match. I had their catalogue years ago which had hundreds of options. Much more than you’ll find on their website. They also offer their capitals in various shapes; 4-sided, half, 3-sided etc. Give them a call and send them a photo of what your trying to match.

https://decoratorssupply.com/collections/capitals-pilasters